Method for operating an internal combustion engine

ABSTRACT

A method for operating a multi-cylinder internal combustion engine having gas exchange valves which are variably adjustable with respect to the valve opening characteristics either directly electromagnetically or by way of an electrohydraulic valve actuation system which includes several electromagnetic valves. For adaptation of all gas exchange valves to a desired valve opening characteristics, the electromagnetic valves are operated cylinder-selectively in the operation of the internal combustion engine by means of variable actuating voltages and/or actuating currents.

The present invention relates to a method for operating an internalcombustion engine according to the preamble of patent claim 1.

In the journal ‘Auto Motor und Sport’, volume 17, 1999, page 49, anelectrohydraulic valve actuation system for an internal combustionengine has already been described which includes a tappet actuated by acamshaft that does not act directly on a gas exchange valve. (inletvalve) in the cylinder head of the internal combustion engine but bymeans of hydraulic oil (motor oil). The pressure of the hydraulic fluidpropagates by way of a brake piston to the gas exchange valve, and thisquantity of hydraulic oil and, thus, also the stroke of the gas exchangevalve in the cylinder head can be varied in response to the valve'sswitch position of an electromagnetic valve integrated in the cylinderhead. Variations of the valve opening times cannot be ruled out due tomanufacturing tolerances for the above-mentioned components so thatunequal cylinder fills will automatically lead also to a spreading ofthe exhaust emissions, especially with respect to the multi-cylinderconstruction of the internal combustion engine.

In view of the above, an object of the present invention is to develop amethod for operating a multi-cylinder internal combustion engine whichpermits adapting the cylinder fills of all engine cylinders so that theabove-mentioned shortcomings are avoided.

According to the present invention, this object is achieved by a methodwith the features of patent claim 1.

Further features, advantages and possible applications of the presentinvention can be taken hereinbelow from the description of an embodimentexplained with reference to several accompanying drawings.

In the drawings,

FIG. 1 is a diagrammatic sketch of an electrohydraulic valve actuationsystem.

FIG. 2 is a view of the variable valve adjustment of the gas exchangevalves resulting from the valve actuation system of FIG. 1, illustratedby several valve stroke curves.

FIG. 3 is an explanation of individual process steps by way of a programflow chart which permit rendering uniform or synchronizing the valvecontrol times of all engine cylinders in consideration of minimumexhaust gas values.

FIG. 4 is a current curve, voltage pulse curve and stroke characteristiccurve, representative of the program flow chart of FIG. 3, for one ofthe electromagnetic valves of the valve actuation or injection system.

FIG. 1 discloses a diagrammatic sketch of an electrohydraulic valveactuation system, including a valve drive unit that is arranged in thecylinder head 1 of an internal combustion engine, comprised of acamshaft 2, a tappet assembly 3, and a gas exchange valve 5 which, inthe capacity of an inlet valve, extends into the intake port 4 of theinternal combustion engine. The gas exchange valve 5 is not actuateddirectly by the tappet assembly 3, but is actuated by means of apressure fluid volume provided by the engine oil pump 6 so as to bevariable with respect to the sequence of motions. To this end, anelectromagnetic valve 7 is inserted into the cylinder head 1 in order tovary the pressure fluid volume compressed between the tappet assembly 3and the gas exchange valve 5. Because the internal combustion engineincludes several cylinders, there is also multiple provision of theinitially mentioned other components of the electrohydraulic valveactuation system corresponding to the number of the gas exchange valves.Besides, the valve actuation system includes per engine cylinder oneintermediate storage 8 that takes up superfluous pressure fluid volume,if any, which is not required for the control of the valve actuation,system. In addition, an injection valve 20 is mounted in the intake port4 which, exactly as the electromagnetic valve 7, can be operated bymeans of a variable actuating voltage and/or a variable actuatingcurrent to adjust all engine cylinders to uniform rates of injection.

The diagrammatic sketch of the electrohydraulic valve actuation systemaccording to the drawings is consequently rated for a multi-cylinderand, thus, multi-valve internal combustion engine in terms of controltechnology, with the objective of influencing the valve strokeelectrohydraulically for each engine cylinder. Only a few millisecondsare available to actuate the electromagnetic valves 7 at high enginespeeds.

A suitable method of operation of the internal combustion engineprevents the system tolerances in the actuation control, the magneticcircuit and the component tolerances within the valve drive unit fromcausing an unacceptable spreading of the valve opening cross-sectionsbecause now the hydraulic control pressure between the tappet assembly 3and the associated gas exchange valve 5, according to the presentinvention, is adjusted individually for each engine cylinder bycontrolling the valve actuating voltage applied to the electromagneticvalve 7 or the valve current so that equal valve strokes for all gasexchange valves 5 result per combustion cycle. Theoretically, this wouldbe technically possible also with the aid of travel sensors in the areaof the gas exchange valves. However, this solution is not feasible dueto cost and structural reasons. Also, care should be taken that theexhaust emission is usually adjusted by means of one single lambda probeper cylinder row.

FIG. 2 shows exemplarily the valve stroke curves which are principallyadjustable by the preset variable valve actuation system according toFIG. 1. Starting from a maximum camshaft angle illustrated on theabscissa, the valve stroke curves are also plotted for reduced valveopening clearances of 40 degrees, 80 degrees, and 120 degrees camshaftangles. Along the ordinate, the valve stroke possible for each camshaftangle is plotted which, automatically, exhibits the smallest valvestroke of roughly 3.8 mm with regard to the smallest camshaft angle of40 degrees.

According to the present invention, FIG. 3 shows the individual processsteps for rendering the valve strokes more uniform and, thus, the valveopening times for all gas exchange valves 5 of a multi-cylinder internalcombustion engine which is preferably equipped with the electrohydraulicvalve actuation system known from FIG. 1. In consideration of theprogram run according to FIG. 3, the above-mentioned system-inducedimponderabilities and tolerances in the actuation control of theelectromagnetic valves 7 and in the valve drive unit may be adjusted sothat each valve actuation system is selectively tuned to an optimum,exhaust emission, with the internal combustion engine running, and theactuation parameters for the electromagnetic valves 7 acquired arestored in a data memory. To this end, the internal combustion engine isfavorably operated in the rotational speed band in which inadmissibledeviations of the exhaust emission of the individual engine cylindersare the result. The exhaust emission is sensed in a per se known mannerby means of a lambda control circuit. The actuating voltage or actuatingcurrents of each single electromagnetic valve 7 is then varied accordingto the program flow chart and stored in the data memory in acylinder-selective manner, and gathered in the way of parameters as afunction of the engine rotational speed. Based on the performance graphof parameters fixed from cylinder to cylinder, the entire actuationcontrol of the electromagnetic valves 7 takes place.

The method for determining the exactly synchronized valve control timeswill now be explained in detail by way of the program flow chartaccording to FIG. 3.

Whenever the internal combustion engine is operated, the electromagneticvalves 7 of all engine cylinders are initialized according to a firstoperation step 9 specifically for adapting the gas exchange valves 5with respect to each other. In a second operation step 10, the worstexhaust gas value is initiated, and the number of the iteration stepsand the iteration step width is determined. In a third operation stepaccording to block 11, it is found out whether the engine speed iswithin a predetermined rotational speed band. When this condition is notsatisfied, a new polling of the engine speed out of the engine controldevice is made by means of loop 11 a. Only if the internal combustionengine is in the predetermined rotational speed band, which isespecially critical in terms of exhaust gas and where a process ofadjustment of the electromagnetic valves 7 shall be carried out, willthe exit to a subroutine according to operation step 12 follow. In saidstep 12, a currently valid and stabilized exhaust gas value is read intoa data memory of the engine control device, what can e.g. be done by wayof linking to a lambda control circuit of the engine management.Subsequently, it is checked in the following block 13 whether thecurrent exhaust gas value is better than the previously stored exhaustgas value. When this request is satisfied, the current actuation valuefor the electromagnetic valve 7 being activated is stored in the nextstep 14 as a function of the engine speed and the associated enginecylinder. If, however, the request for an improved exhaust gas value isnot satisfied after step 13, the iteration method and, hence, the valveadjustment for the currently concerned engine cylinder is continued byway of loop 14 a instead of step 14. It is checked in operation step 16whether all iteration steps have been processed. Unless all iterationsteps have been processed, the valve adjustment process will be repeatedstarting from block 2 by way of loop 16 a. If, however, all iterationsteps have been completed, the next electromagnetic valve 7 will bepicked up according to field 17. It is checked in step 18 whether theelectromagnetic valves 7 of all engine cylinders are adapted. In thenegative, the sequence diagram is then repeated commencing operationstep 10 by way of loop 18 a. If, however, the adaptation of all enginecylinders is completed, the valve adaptation method explained isterminated with step 19.

When this valve adaptation process for the individual engine cylindersis gathered by an appropriate algorithm, the offset of the valve controltimes in relation to a nominal specification, i.e., determining onlycrankshaft angles of rotations, may be determined in a comparativelysimple manner in order to adjust the optimizing parameter ‘exhaust gasquality’ in this case.

In an extension of the basic idea, values for different rotational speedranges can be determined and stored in a data memory of the enginemanagement or engine control device. A performance graph or a set ofparameters for a mathematically description may be found out thereby.

The algorithm can be used in a test run to determine the parameters. Inaddition, the algorithm may also be used in the normal operating mode ofthe internal combustion engine in order to optimize the parameters, e.g.to counteract the aging of components. To this end, it would benecessary to modify the operation step 2 according to FIG. 3 and toindicate the engine speed as an index in the performance graph.

To sum up, a method for operating an internal combustion engine is shownwhich permits optimizing the exhaust gas values by variation of theactuation times of the electromagnetic valves 7 and, hence, thesynchronous actuation of the gas exchange valves 5 (inlet valves). Thisis done by varying the actuation parameters of the electromagneticvalves 7 in a search operation described in FIG. 3. The result is anoptimal valve actuation control for a quality criterion or also forseveral quality criteria.

With regard to the program flow chart according to FIG. 3, an optimizedcurrent characteristic curve for each engine cylinder will resultaccording to FIG. 4 for the electromagnetic valve 7 being respectivelyactivated, wherein the optimal current variation is determined as afunction of time and, hence, proportionally to the engine crank angle aswell as by the trigger point T. The result of the,adaptation processaccording to the present invention is a saw-tooth current variationcharacteristic curve which commences with a comparatively low deadcurrent I1 (starting current), which along with the rise to the excitingcurrent I2 simultaneously causes movement of the magnet armature of theelectromagnetic valve 7 and keeps it in the open position until thetrigger point T is reached due to the decrease of the exciting currentI2 to the holding current I3 which, in its amount, is slightly higherthan the dead current I1, with the result that the magnet armature ofthe electromagnetic valve 7 moves to resume its original inactiveposition. Due to the method illustrated in FIG. 3, the trigger point Tis gathered in a data memory of the engine control device for eachelectromagnetic valve 7 and, thus, for each gas exchange valve 5 in theengine cylinder. The time variation of the current pulse and themovements of the magnet armature are phase-identically plotted below thecurrent characteristic curve, whereby a direct allocation of the currentpulse duration and the magnet armature movement to the currentcharacteristic curve is rendered possible.

To sum up, a valve actuation method is achieved wherein the exhaustemission is measured for each engine cylinder, and whereinsubsequently—with the objective of reaching optimized exhaust gasvalues—the actuating voltage or the actuating current is alternatinglyvaried as a function of the engine crank angle for each electromagneticvalve 7, and the optimal trigger point T is determined. The optimalswitch points of the electromagnetic valves 7 determined during theprocess are thus acquired individually for each engine cylinder andmemorized as a field of parameters in the data memory of the enginecontrol device as a function of the engine speed. Based on this fixedfield of parameters, a cylinder-selective valve actuation control willthus be effected which, in the present example, finally leads to equalvalve strokes of the gas exchange valves 5.

It is, however, not absolutely necessary that the valve strokes of theelectromagnetic valves 7 are equal. Instead, they may be varied inconformity to any requirement and request with a view to achieving theobjective. According to the above valve control method, the tolerancesof the rate of injection can also be adjusted by a cylinder-selectiveactuation of the injection valves 20.

The present invention is not restricted to the constructive embodimentof FIG. 1 but also appropriate for alternative valve drive constructionswhich, for example, arrange for a direct electromagnetic actuation ofthe gas exchange valves and either include manifold injection or directinjection.

List of Reference Numerals:

1 cylinder head

2 cam shaft

3 tappet assembly

4 intake port

5 gas exchange valve

6 engine oil pump

7 electromagnetic valve

8 intermediate storage

9-19 operation steps

20 injection valve

What is claimed is:
 1. Method for operating a multi-cylinder internalcombustion engine having gas exchange valves which are variablyadjustable with respect to an opening characteristic of the gas exchangevalves by way of a direct electromagnet or by way of an electrohydraulicvalve actuation system which includes several electromagnetic valves,wherein the electromagnetic valves for adaptation of all gas exchangevalves to a desired valve opening characteristics are operatedcylinder-selectively in the operation of the internal combustion engineby way of variable actuating voltages or actuating currents, wherein theadaptation of the valve opening characteristics of all gas exchangevalves is preferably effected in a data memory of an engine controldevice, comprising the steps of: a) initializing the number of theelectromagnetic valves or injection valves of all engine cylinders, b)initializing an the exhaust gas value with the highest degree ofemission and defining a number of iteration steps and an iteration stepwidth, c) checking whether a predetermined engine speed is maintainedwhere an inadmissible cylinder-selective spreading of the exhaust gasvalues is to be expected, d) reading the currently valid and stabilizedexhaust gas value into a data memory, e) comparing the current exhaustgas value with the exhaust gas value that was previously stored in aperformance graph of the data memory, f) storing the current exhaust gasvalue in the data memory in dependence on an engine speed, if thecurrent exhaust gas value is better than the original exhaust gas valuestored in the performance graph of the data memory, g) continuingiteration steps and adjusting the electromagnetic valves, h) storing oneor more actuation parameters of all electromagnetic valves in the datamemory.
 2. Method as claimed in claim 1, wherein the adaptation of allelectromagnetic valves is carried out in dependence on thecylinder-selective exhaust emissions of the internal combustion engine.